March 15, 2013

Bizarre Whale Strandings Not Due To Family Ties, True Cause Remains A Mystery

Since the time of Aristotle, biologists have struggled to understand the reasons for mass strandings of whales and dolphins on beaches. Contrary to previous assumptions that whales follow each other onto the beach — and almost certain death — on account of familial ties, a new study from Oregon State University and the University of Auckland reveals that many unrelated individuals are present at each event.

One theory that seeks to explain these strandings is the idea that "care-giving behavior," mediated by large family groups, plays a critical role. Thus the stranding of one or a few whales because of sickness or disorientation, according to the old theory, triggers a chain reaction in which healthy whales are drawn into the shallows in attempts to support family members.

The new study questions this theory using genetic data to describe the familial relationships of individual long-finned pilot whales that were involved in mass strandings in New Zealand and Tasmania, the largest of which included more than 150 whales. All of the whales in that incident died.

Stranded groups are not necessarily members of a single extended family, the study found. This contradicts the hypothesis that members of stranded groups all descend from a single ancestral mother.

In many cases, genetic identification showed that the mothers of calves were missing entirely from groups of whales that died in the stranding, said Scott Baker, associate director of the Marine Mammal Institute at Oregon State University. This suggests that strong kinship bonds are being disrupted prior to the actual stranding. Baker and his colleagues suspect that this disruption might play a role in causing the event.

"Observations of unusual social behavior by groups of whales prior to stranding support this explanation," said Baker, who frequently advises the International Whaling Commission (IWC) and is a professor in the Department of Fisheries and Wildlife at the university's Hatfield Marine Science Center.

The most common species to strand en masse is the long-finned pilot whale. Scientists have long assumed that this tendency was somehow connected to the species' social organization. Prior studies have revealed that pilot whales have a matrilineal social organization in which neither males nor females disperse from the group in which they were born. This group structure is otherwise thought to be rare in mammals, although it is also seen in killer whales.

Marc Oremus of the University of Auckland says the mass stranding of pilot whales is common in New Zealand and Australia, involving several thousand deaths over the last few decades. The research team performed genetic analysis of 490 individual pilot whales from 12 separate strandings. They examined both mitochondrial DNA, which is inherited exclusively from the mother, and microsatellite genotypes, which are inherited from both parents. This analysis showed multiple maternal lineages among the victims in each stranding, and thus no correlation between kinship and the grouping of whales on the beach.

"If kinship-based social dynamics were playing a critical role in these pilot whale strandings, first, we would expect to find that the individuals in a stranding event are, in fact, all related to each other. Second, we would expect that close relatives, especially mothers and calves, would be found in close proximity to each other when they end up on the beach during a stranding event," explained Oremus.

"It is usually assumed that environmental factors, such as weather or the pursuit of prey, brings pilot whales into shallow water where they become disoriented," Baker said. "Our results suggest that some form of social disruption also contributes to the tendency to strand."

"It could be mating interaction or competition with other pods of whales," Baker continued. "We just don't know. But it is certainly something that warrants further investigation."

The researchers assessed the spatial relationships of individual whales on the beach in some strandings, mapping the position of each animal to determine if individuals found near each other were related. Even when considering only the location of nursing calves and their mothers, who were often widely separated when the group drove itself onto the shore, they found no correlation between location and kinship.

The evidence of "missing mothers" was the most surprising. Many stranded calves and juveniles had no identifiable mother among the other beached whales.

"Several scenarios could account for the lack of spatial cohesion, including the disruption of social bonds among kin before the actual strandings," commented Oremus. "In fact, the separation of related whales might actually be a contributing causal factor in the strandings, rather than simply a consequence."

This has important implications for rescue efforts of the stranded animals.

"Rescue efforts aimed at 'refloating' stranded whales often focus on placing stranded calves with the nearest mature females, on the assumption that the closest adult female is the mother," Baker pointed out. "Well-intentioned rescuers hope that refloating a mother and calf together will prevent re-stranding. Unfortunately, the nearest female might not be the mother of the calf. Our results suggest that rescuers should be cautious when making difficult welfare decisions — such as the choice to rescue or euthanize a calf — based on this assumption alone."

An important remaining question is: where are the "missing mothers?" Were the females able to refloat or were they somehow able to avoid getting stranded to begin with? The research team hopes that their study will lead to further genetic sampling of pilot whales and other stranded whale species, as they use satellite tags to monitor the survival and behavior of whales that are helped back into the ocean.

"The causal mechanisms of these strandings remain an enigma," Oremus said, "so the more avenues of research we can pursue before and after the whales beach themselves, the more likely we are to discover why it happens."